Cathodic protection by coating for cooling circuits or other holes or channels

ABSTRACT

The present invention develops dies, moulds, tools, workpieces and structures with active protection against corrosion (cathodic protection) in cooling circuits, often internal, independent of its complexity. Moreover, the protection can be renewed if considered appropriate. The protection is achieved through a thin coating layer which contains metallic elements more electronegative than the substrate to be protected and which is properly adhered to the mentioned substrate in order to ensure an efficient cathodic protection.

FIELD OF THE INVENTION

The aim of the invention is to obtain an electronegative coating with respect to the material being coated which provides cathodic protection (active protection against corrosion) for complex holes geometries of any kind of workpiece, part or tool, in particular, for cooling channels of tools which come into contact with aggressive media, and any cooling liquid, specially water. The present invention includes both the novel technique for preparing and coating the holes, shells and other geometries as well as the resulting product.

DESCRIPTION

In general, moulds used in some particular forming process where it is convenient to evacuate heat from the tool, such as in injection moulding, either of light alloys or of plastic, are made of hot work tool steel, having an internal cooling system with channels through which a cooling liquid circulates. The mentioned internal cooling system is of vital importance for the mould, as it allows the extraction of heat more efficiently and it prevents overheating during the different forming process stages.

There are great deals of other technical applications where a compromise between mechanical and/or physical properties is to be reached, so that the use of metallic materials with limited corrosion resistance to some aggressive media is recommended. Often in such processes, a thermal control or heat evacuation through a circuit which acts as a heat exchanger is required. Often the coolant used becomes an aggressive media for the metallic material used. In other occasions the metallic material has to come into contact with aggressive process mediums (aggressive process liquid conduction, aggressive gas shut-off, immersion in aggressive medium . . . ). In some of these cases, metallic materials which meet the required mechanical properties as well as being also resistant to the medium can be used. But there are a lot of applications where the best compromise among the required mechanical and/or physical properties can be improved if no aggressive media resistance is to be achieved; in other cases, and if done in an integral way, the obtaining of this medium resistance leads to a large cost increase. In all of these cases, it would be very convenient the use of FGM solutions type (Functionally Graded Materials)—materials with functionality spread out over more than one layer. Thus, it would be very convenient to provide resistance to medium through a protective layer.

For most of these applications, it is also desirable that the protective layer offers cathodic protection to the base material, so that even if a defect in the protective layer appears, the exposed underlying base material remains protected from corrosion due to the consumption of the protective layer. If the protective layer can be periodically rebuilt to compensate its eventual consumption, the solution becomes much more complete.

There are a lot of applications where this kind of functionality gradation against the medium is used: galvanized sheets, galvanised wires, rustless paints . . .

Some of these solutions offer cathodic protection to the underlying base material, such as galvanizing, where an immersion in the molten protective metal or a galvanic coating provides a metallic layer more electronegative than the base metal and acts as sacrificial anode. The problem is that all of these methods were developed and are used only for the protection of open surfaces and not for the protection of cooling or conduction systems, especially when closed circuits have a small section and long length.

The most commonly method used to protect these cavities from corrosion is to chemically treat the coolant so that it is no longer aggressive for the material which remains into contact with. This treatment has the disadvantages of being expensive and of needing continuous maintenance.

Alternatively, and to avoid the risk of corrosion in those zones where once damaged they can cause danger situations during the forming process and/or irreparable damage in the mould, stainless steel is used, although it is not the suitable material for hot work forming, as it exhibits worse properties than hot work tool steels. Even chlorine used for the treatment of cooling water in stainless steel dies can attack the material, especially when working at high temperatures.

Another current alternative is to protect the cooling channels using a coating of chemical Ni, which protects the walls of the channels as it does not exhibit corrosion when coming into contact with the cooling liquid. This method does not work anymore when the coating has a defect, either coming from the method itself during the application of Ni or due to die operation, or because of the apparition and propagation of cracks. When the Ni layer disappears, the steel remains exposed to the cooling liquid and steel corrosion is even more intense, since Ni acts as a powerful cathode with respect to the steel and this is the material which is consumed for being the anodic surface smaller. Thus, the material intended to protect ends up being attacked in a local and aggressive way.

Regardless of the method used to generate the protective layer, another problem is found concerning the interaction between the protective layer and the underlying metal. Although sometimes it is not visible, surfaces from most of metals are covered with soil, namely, oil, rust, grease, . . . These thin films hinder the adherence of both materials resulting in a weak, and in some cases even inexistent, direct bond between the protective layer and the metal being protected. This bad interaction makes the electron transference between both materials difficult and therefore, in case of an undesirable defect in the protective layer, cathodic protection is no longer effective and after some time the underlying metal ends up corroded even when the coating contains elements more electronegative than the base material.

The present invention uses the sol-gel process to apply the cathodic protection technique within the inner walls of cavities of a workpiece, Fe alloy or cooling circuit. This technique is used to control the corrosion of a metal surface, turning it into the cathode of an electrochemical cell, such as for example, the channel inner walls of a hot work tool steel cooling system.

The cathodic protection is induced through the connexion of the metal being protected with another metal easier to corrode, since it will work as a sacrifice anode from the electrochemical cell, composed from two metals in common contact in a wet medium, for example cooling water. As it was already mentioned, to assure an effective protection, both metals have to share a chemical bond in order to allow an electron flow from the anode to the cathode. Therefore it is convenient to activate the surface of the metal being protected. This can be done through many different methods: one example is pickling the surface of the metal being protected with a removal substance. After it, it is suggested to treat the surface with a solution which removes all acid, rust or any kind of rest and convenient to wash with a universal solvent. After this process, the lacquer can be directly applied.

Under these conditions, when applying an electronegative coating with respect to the underlying material being protected and in case of defect or damage in the coating leaving the material in contact with the electrolyte, the material being corroded is the coating, that means that the coating is consumed protecting the workpiece integrity.

Mg, Al and Zn are three examples of electronegative elements with respect to steel, which are used in the preparation process of a liquid solution, sol-gel lacquer, and which contains particles of the mentioned elements alone or in combination. After curing, the sol-gel becomes a solid phase allowing the formation of a metallic layer from the mentioned elements which covers all inner surfaces of cavities being protected.

As a matter of fact, any element which is electronegative with respect to steel can be used for this purpose.

The process starts with the filling of the cavity with the sol-gel lacquer, applying pressure to force its adhesion with the walls of the holes. After it, curing may take place due to the application of temperature, pressure or another activation method, such as the use of a catalyst or sound waves. Contraction and densification of the metallic part of the coating take place during this stage. The final coating microstructure, more or less dense, depends significantly on the curing process.

The main advantage of this deposition method is the facility with which it can be moulded, as the initial solution turns gradually into a biphasic system of jelly-like consistency. This process is also advantageous because it is cheap and allows precise control of the final product chemical composition.

Another method used as well for protection against corrosion is the application of a typical galvanic coating. Using the same starting elements as for the sol-gel lacquer, Zn, Al and/or Mg, an electronegative coating with respect to Fe is obtained. Afterwards the metallic coating can be applied with the already known traditional galvanic treatments, such as hot-dip galvanised, electroplating . . .

Good results can also be achieved using the electrostatic coating method where a more uniform deposition is achieved. The lacquer is electrostatically charged and sprayed onto the polarized substrate, considering that polarisation of the substrate decreases with the increasing deposition of the protective layer and therefore, particles are always deposited in those parts were the layer is thinner. This method of application is highly recommended for protecting shells, small parts or short length cooling channel parts.

In the present invention, and with the aim to cover the inner part of holes with this cathodic protection layer, an electrolyte has to be flown in the whole circuit, with anodic charge and enough pressure to assure an effective and homogeneous adhesion of the anode ions through the entire path being coated. This technique shows limitations concerning the cavity length and the desired layer thickness. The more is the length of the cavity to be coated, the less is the penetration. Thus, this technique is just suitable for easy geometries circuits and limited lengths.

STATE OF THE ART

Up to now, the different galvanic coating methods for empty or concave bodies, referenced in for example DE 10308731 A1 or CN 101302636 A, refer to the application of a protective layer or submerging the entire workpiece, and therefore coating the whole workpiece, or by protecting the parts not desired to be coated with an anti-galvanic membrane. Other presented patents, for example CN 1542168 A or JP 2008156685 A, solely address to partially coating of cylindrical bodies or workpieces. In any case it is considered how to cover holes or workpiece cavities with galvanic treatments.

Other several methods are also known for applying a coating, for example from Zn or Zn alloys with other metals such as Ni or Mg, through hardening and tempering by cast immersion or electrolytic separation in metal sheets with the aim of protecting them against corrosion, as it is described in DE 19527515 D1. The described method in the last mentioned invention is just applicable to the coating for inner or outer surfaces that can be easily reached through the immersion of the whole workpiece in the electrolytic bath.

There are also other inventions, for example DE 19829768 C1 or US 2004140220 A1, which describe the composition of an electrolyte so that it can be used as a coating for steels to improve their corrosion resistance. In particular, the DE 19829768 details the dissolution process of Zn coming from the Zn granules in a ZnNi electrolyte weakly acidic, for its use as a galvanic covering, which results in a more corrosion resistant coating in contrast to a pure Zn coating. Or in the case of US 2004140220 A1, which describes a process to galvanize a metal with an electrolyte of Al, Mg or combinations of both of them or other elements, with the aim of covering the material with a protecting layer against corrosion. Chemical compositions of the last two mentioned inventions also focus on the coating of metallic surfaces through galvanisation, specially for laminated steel sheets.

US 20110017361 A1 describes the method employed to cover steel sheets with a metallic covering which protects them from corrosion attacks in aggressive mediums. This method consists on submerging the steel sheet, previously pickled, in a molten metal bath (Zn, Al, Mg or some of its combinations, such as Zn+Al or Zn+Mg) to provide a resulting electronegative coating with respect to the underlying material, and protecting it from corrosion.

Although the purpose of the described coating in the already mentioned patent is the same as the one of the present invention, to give cathodic protection to an element, the method used to apply it and the desired geometry to be protected have no point in common. US 20110017361 A1 suggests the application of the coating in outer surfaces through a hot galvanic method, while in the present invention the coating is suggested to be provided by circulating a metallic salts solution which carries metallic ions which will deposit in the inner surface of the metal to be protected.

US20100175794 describes a method for the application of lacquers with active protection against corrosion for metal sheets in the hot stamping process. In the present invention, products applicable through wet chemical methods similar to the cited document can be employed.

US20080265217 A1 also describes a way of using cathodic protection and describes a type of compounds based on metallic particles less noble than the metal in the metal substrate to be protected containing a conductive polymer.

DETAILED DESCRIPTION OF THE INVENTION

Cathodic protection to inner circuits from a workpiece, die, mould, tool or structure under study of the present invention can be achieved by means of different ways.

The desired method for applying the protective layer is by means of wet chemistry. Especially, by the use of lacquers which contain particles or pigments from electronegative elements or alloys (in respect with the steel) and conductor elements which allow the electronegative metallic elements work as sacrificial anodes, to actively protect the underlying material. An optimal technique is the use of the sol-gel technology.

To generate the protective layer, the easiest way is to fill the holes or inner circuits being protected with the protective lacquer and use temperature to dry it. But at the end, many other procedures can be used in order to make the protective layer reach the surfaces to be protected, as for example, recirculating the lacquer, alone or diluted by the circuit itself, by means of using an ink pad or similar . . . Also the method for fixing the protective layer to the surfaces to be protected has several varieties: using temperature, surface activation, pressure, ultrasounds or other waves, etc.

Actually, when incorporating the lacquer into the cooling fluid from a closed system, it can be recirculated and therefore, its application will be continuous. In this particular case, it is not necessary to cure with temperature to dry the lacquer on the walls being protected.

As it was previously introduced, an advantageous execution of the present invention relates to an effective cathodic protection thanks to a good electron transfer, and therefore a strong chemical bond, between the metal being protected and the generated protective layer. To achieve this high level of interconnection, it is desirable to activate the surface of the metal being protected. It is very common to find some traces of oil, grease, rust or any other disturbing thin layers on the surface of the metal to be protected, even if sometimes it is not visible with the naked eye. In these cases or in many other circumstances, and depending on the layer to be removed, one can use different methods to activate the surface of the material being protected. For instance, it can be used an acidic etching (to remove for example a rust layer, among others, . . . ), inorganic and/or organic solutions (if the surface has some oil and/or grease and/or other dirty traces, among others . . . ), pickling (in cases of strong dirtiness and/or small to extreme amounts of oxide, among others . . . ), . . . and so on; the mentioned solutions and or compounds can be used in different concentrations, alone or diluted (with water or other solvents);

some examples are HCl, HNO₃, HF . . . among others, alone or mixed. So the type of solution used in this first step depends on one side on the layer to be removed. But in some cases it is also important to consider the metal of the substrate being protected, as some of the activating solutions are very strong and can damage the underlying metal. Thus, although the removal of an undesirable layer/layers would be effective, protection would fail because of a premature deteriorating of the substrate. On the contrary, it can also happen that the activation solution is not strong enough and thee base metal remains uncleaned.

Another problem can come when using some of the mentioned acidic solutions. It can happen that some acidic traces remain in the substrate and in this case, if the lacquer is applied immediately after the mentioned activation step, the remaining acidic ions can deteriorate the substrate after some time. Thus, in one hand it would lead to a high union between the two layers but after some time, the underlying metal would end up corroded as well due to the trapped acidic remains. Concerning this fact, it is very advisable to control the aggressiveness of the medium before and after eliminating the dirty layer. It is recommended to use a neutralising solution after it, or a rust-off organic or inorganic solution, or any other organic or inorganic solution amongst others. It is therefore very important to assure a properly activated and passivated surface of the metal being protected in order to guarantee that it does not deteriorate with time.

Another important point relates to the time spent between the surface is activated and the lacquer is applied. After the mentioned surface activation, the free surface of the metal being protected is prone to react with oxygen and oxidise. Thus, it is preferred to apply the lacquer after the minimum time has elapsed from activation. It is very convenient less than one day, preferably less than two hours, more preferably less than one hour and even less than half an hour from activation.

The present invention offers active protection against corrosion in cooling circuits, even if the coating is not perfectly applied in some specific area depending on the method used, and even if the coating is scratched during its life service due to any stage of its operation, and even if any other undesirable circumstance causes or induces any kind of defect along the protective layer. If surface is not activated as it is stated in the present invention, active protection against corrosion cannot be effective even if the protective layer is more electronegative than the underlying metal being protected.

The present invention can also be used for the protection of cooling channels of workpieces, moulds or dies, which are machined in shells and closed afterwards using rubber gaskets or strong welding (brazing). When using rubber gaskets, the lacquer has to be applied before the assembly; on the other hand, when the joint is done by strong welding, the lacquer is suggested to be applied after the joining,

The present invention is specially indicated for the protection of cooling systems of plastic injection moulds, aluminium injection moulds, hot stamping dies or any other forming process which may require heat dissipation through a system which works as a heat exchanger.

It is especially advisable that the material used for the cathodic protection is a mixture containing Al, Zn, Mg and/or Sn embedded particles or pigments. The embedded particles can be also alloys from the above mentioned elements as long as they are electronegative in respect with the substrate being protected in order to be able to offer active protection against corrosion.

EXAMPLES Example-1

A hot stamping die segment made of high thermal conductivity tool steel, like HTCS, with a cooling system of parallel holes of 8 mm diameter and 12 mm from the surface, was coated via wet chemical method. The channels were filled with a lacquer containing Zn and Al pigments. The segment with the already filled cooling channels was introduced in a furnace and hold at 200° C. for half an hour. Afterwards, the ends of the filled holes were released to remove the non-hardened lacquer. A hardened and uniform film from approximately 30 micrometers covering the cooling channels was obtained. This segment was mounted adjoining to another part without treatment. After seven months the channels were investigated using an endoscopic camera. Big amounts of red oxide and a strong corroded surface with deep cavities partially full from chalky waste were observed. On the contrary, the protected segment showed an oxide-free surface with the original roughness it had at the moment of the protective layer application.

Example-2

Three holes from 300 mm length were drilled in a rectangular workpiece made of steel for plastic injection with moderate corrosion resistance, type W. Nr-1.2738 and dimensions 300×200×150. The first drill hole was treated like the previous example. The second hole was treated with a sol-gel lacquer with Zn pigments, and it was also hardened by drying at 180° C. for two hours. The third hole was not treated. The workpiece was introduced in a salt spray chamber and after 25 days it was analysed again with an endoscopic camera. The workpiece showed big amounts of iron oxide and a severe attacked surface in all free surfaces, particularly at the beginning of the non-treated hole. Also the walls of the non-treated hole showed corrosion, although not as much as the free surfaces. None of the treated holes showed corrosion.

Example-3

A hot stamping die segment made of high thermal conductivity tool steel, like HTCS, with a cooling system of parallel through-holes from 8, 10 and 12 mm diameter respectively, and 30 mm in length, was investigated under a corrosive environment by Salt Spray test (ASTM 117). Different sets of holes were treated in order to activate the surface before the application of the sol-gel lacquer, and for another different set of holes the lacquer was applied directly to the substrate. The lacquer in both cases was cured at 180° C. during 45 minutes. The salt spray test was carried out under the following experimental settings: 50 g/l NaCl, pH=6.5-7.2 and T=35° C. Inspection at the interior of the cooling channels was done with an endoscopic camera, as-received and at regular periods of time (24, 48, 96 and 168 hours) after the corresponding treatment. Results show a bad behaviour of the samples without surface activation at the earliest time of exposure whiles the samples with a previous removal of oxide and grease showed no signal of corrosion. Neither did it after 48 nor 96 hours. First signs of oxidation began to be seen after 196 hours.

Example-4

New different sets of holes with the same characteristics as in example 3 were investigated under a corrosive environment with water. Samples were treated in three different ways. For the first treatment, samples were pickled with HCl and coated with the sol-gel lacquer. At the beginning, they did show a good resistance, but after one month, corrosion was observed under the layer of the lacquer, corroding the underlying base material. For the second inspection, samples were pickled again with HCl, neutralised (in order to passivate the base material) and coated. These samples showed a stronger corrosion resistance and no oxidation was observed between the two layers during eight months. Even after some time, samples were scratched and left again in water and still did not corrode during four months. For the third investigation, samples were not pickled and directly coated on the base material. These samples presented good corrosion resistance during several months but corroded immediately after after a few days when scratching them.

Additional embodiments of the invention are disclosed in the dependent claims. 

1. Steel-based mould, die, tool, workpiece or structure comprising cooling circuits and/or holes, characterised in that the cooling circuits and/or holes are at least partly coated with at least a material offering cathodic protection to the underlying substrate.
 2. Steel-based mould, die, tool, workpiece or structure according to claim 1, wherein the material offering cathodic protection to the substrate comprises a mixture containing embedded particles or pigments made of at least one of Al, Zn, Mg and/or Sn.
 3. Steel-based mould, die, tool, workpiece or structure according to claim 2, wherein the embedded particles are made of alloys of at least one of Al, Zn, Mg and/or Sn, provided that these alloys are electronegative with respect to the underlying substrate in order to offer active protection against corrosion.
 4. Steel based mould, die, tool, workpiece or structure according to claim 1, wherein the material offering cathodic protection to the substrate is a sol-gel lacquer.
 5. Steel based mould, die, tool, workpiece or structure according to claim 1, wherein the underlying substrate comprises a high thermal conductivity steel with a conductivity higher than 42 W/mK.
 6. Method of manufacturing steel-based moulds, dies, tools, workpieces or structures having cooling systems and/or holes, which method comprises the step of at least partly coating the cooling systems and/or holes with a coating offering cathodic protection to the underlying substrate.
 7. Method according to claim 6, wherein the underlying substrate is coated by means of a wet chemical method.
 8. Method according to claim 6, wherein the underlying substrate is coated using a sol-gel lacquer.
 9. Method according to claim 8, wherein the surface of the underlying substrate is activated prior to the application of the sol-gel lacquer.
 10. Method according to claim 9, wherein the surface of the underlying substrate is activated by picking.
 11. Method according to claim 9, wherein the surface of the underlying substrate is activated by etching.
 12. Method according to claim 9, wherein the surface of the underlying substrate is activated using HCl.
 13. Method according to claim 9, wherein the surface of the underlying substrate is activated using HNO₃.
 14. Method according to claim 9, wherein the surface of the underlying substrate is activated using HF.
 15. Method according to claim 6, comprising the following steps: activating the surface of the underlying substrate with a media aggressive for said surface; neutralizing the rests of the aggressive media; and applying of a sol-gel lacquer to the activated surface.
 16. Method according to claim 15, wherein, after the neutralization step, the rests of the aggressive media are removed with an organic solution.
 17. Method according to claim 15, wherein, after the neutralization step, the rests of the aggressive media are removed with an inorganic solution.
 18. Method according to claim 16, wherein, after the removal of the rests of the aggressive media, the surface of the substrate is washed with a universal solvent.
 19. Method according to claim 9, wherein the lacquer is applied within one day after the surface activation.
 20. Method according to claim 9, wherein the lacquer is applied within two hours after the surface activation.
 21. Method according to claim 9, wherein the lacquer is applied within one hour after the surface activation.
 22. Method according to claim 9, wherein the lacquer is applied within half an hour after the surface activation.
 23. Method according to claim 6, further comprising a drying step.
 24. Method according to claim 23, wherein the holes are filled with the sol-gel lacquer during the drying step.
 25. Method according to claim 23, wherein the lacquer, either undiluted or diluted, is recirculated through the holes during the drying step.
 26. The method of claim 6, comprising plastic injection.
 27. The method of claim 6, comprising aluminium injection.
 28. The method of claim 6, comprising hot plate stamping. 